Field of the Invention
The invention generally relates to an optical apparatus and an element thereof, and more particularly, to a projection apparatus and a projection lens thereof.
Description of Related Art
Usually, if a projector is used to project an image to a larger screen, a longer projection distance is required. Contrarily, if projector is used to project an image with a shorter projection distance, a specific wide-angle lens is required to shorten the distance between the projection screen and the projector.
In the design of a wide-angle lens, aberration is a difficult issue a designer needs to be confronted with. In order to reduce the focal length and the aberration, many optical devices should be disposed in the projector. U.S. Pat. No. 7,567,380 discloses a projection lens using spheric lenses and aspheric lenses to refract light beams. All of elements to transmit the light beams are lenses in this patent. On the other hand, a designer adopts a plurality of reflection minors to shorten the projection distance, as shown by U.S. Pat. No. 7,717,573. However, the increasing reflection mirrors would increase the production cost and assembly difficulty.
Furthermore, U.S. Pat. No. 8,054,541 discloses a projection lens adopting a plurality of refraction lens at the object side and one reflection mirror at the image side to project images onto a screen.
Moreover, US Pub. No. 20090116124 discloses a projection optical system 50 including a first refracting optical section 20 having a triple-cemented lens L3 and a reflecting optical section 30 having a curved mirror 31. However, many optical properties of the projection optical system 50 should be improved.